The following copending, commonly assigned United States Patent Applications contain disclosure pertinent to the present inventions: Ser. No. 09/138,858, filed Aug. 24, 1998; Ser. No. 08/440,665 filed May 15, 1995, now U.S. Pat. No. 5,801,441; Ser. No. 08/271,768, filed on Jul. 7, 1994, now U.S. Pat. No. 5,518,964; Ser. No. 09/095,251, filed Jun. 10, 1998; Ser. No. 08/532,528 filed Sep. 22, 1995, now U.S. Pat. No. 5,798,286; and Ser. No. 08/690,532 filed Jul. 31, 1996. The disclosures of said applications and patents are incorporated by reference herein. The disclosure of U.S. Pat. No. 5,148,266 is also incorporated by reference herein.
A copending, commonly assigned United States Patent Application entitled Framed Sheet Processing, which application claims benefit of said United States Provisional Patent Application 60/061,932 is being filed of even date herewith. Said application filed of even date herewith is hereby incorporated by reference herein.
The present invention relates to fabrication of microelectronic circuit components, interconnections and packages, and to articles useful in such processes.
Many microelectronic assemblies employ panel-like circuit elements. For example, one common method of connecting the contacts on a semiconductor chip to external circuitry, referred to as tape automated bonding or xe2x80x9cTABxe2x80x9d uses a sheet-like tape including a flexible dielectric layer, typically polyimide with metallic circuit traces thereon. Ordinarily, these circuit traces are formed by photochemical processes such as etching or plating using photographically patterned resists. The precision with which such a circuit can be formed is limited by the dimensional stability of the dielectric layer during processing. This problem increases as the size of the circuit increases. Typical TAB tape has numerous individual circuits made by photographically patterning an area of the flexible circuit of the size required to mount a single chip. The individual circuits are spaced along the length of the chip. Because the process only requires registration of features over a relatively small region corresponding to the dimensions of an individual chip, there is no need to maintain precise spacing between widely separated features. Moreover, typical TAB tapes do not require especially precise registration between features formed in different stages of the manufacturing process.
Larger circuits which require precise relative location of widely-spaced features have been fabricated heretofore using a xe2x80x9cdecalxe2x80x9d or xe2x80x9cappliquexe2x80x9d approach in which the flexible circuit is fabricated on the surface of a metallic plate. The metallic plate is then removed, as by exposure to a caustic etching process. For example, certain embodiments of U.S. Pat. No. 5,055,907 disclose manufacture of a large circuit on the surface of an aluminum plate. After fabrication of the circuit, and after microelectronic elements such as individual semiconductor chips are mounted to the circuit, a support ring is attached around the periphery of the circuit and the plate is removed. In this arrangement, the plate maintains dimensional stability of the circuit throughout the fabrication and mounting process. However, the additional process steps required to remove the plate considerably complicate use of this approach. Also, the plate precludes access to one side of the circuit which impedes the fabrication process and restricts the design of the finished product.
The aforementioned U.S. Pat. No. 5,518,964 and the corresponding PCT International Publication WO 96/02068, the disclosure of which is also hereby incorporated by reference herein, disclose processes in which circuit elements such as microelectronic connection components are fabricated in the form of a wafer-size sheet. In certain processes disclosed in the ""964 patent, a sheet of a starting material such as a flexible dielectric sheet with metallic layers thereon is stretched and bonded to a rigid frame having an opening or aperture therein so that the sheet is held taut by the rigid frame and maintained under tension by the frame. The frame may be in the form of a ring. The ring may be formed from a material such as molybdenum, which has a coefficient of thermal expansion close to that of a silicon semiconductor wafer, and lower than the coefficient of expansion of the sheet. The sheet may be stretched and attached as by bonding to the ring at an elevated temperature, so that the sheet remains in tension during processing at lower temperatures. While the sheet is held in the ring, it is accessible from both sides. The sheet is treated using various circuit-fabrication techniques such as etching and plating using photographically patterned resists. Because the sheet is maintained under tension throughout the process, it remains dimensionally stable. Because the sheet is accessible from both sides, fabrication of the sheet, and mounting of the sheet to the wafer can be performed readily. The features formed on the sheet are precisely positioned relative to one another over the entire extent of the sheet.
After processing, the entire sheet, with the rings still attached, can be aligned with a large assemblage of semiconductor chips such as a unitary semiconductor wafer. Leads formed during the fabrication process can be connected to all of the chips on the wafer. After connection, and after other processes such as deformation of leads on the sheet and injection of curable compliant material, the individual chips and associated portions of the sheet can be severed to provide individual packaged chips or subassemblies, each including one or more chips and an associated part of the sheet. Thus, the basic approach of using a rigid frame around the periphery of a sheet to provide dimensional stability during fabrication, as set forth in the ""964 patent, allows fabrication of microelectronic circuit elements in large array, such as wafer-size arrays with excellent dimensional stability and control. Further improvements in this basic approach are taught in the aforementioned commonly assigned U.S. patent application Ser. No. 08/690,532 filed Jul. 31, 1996, entitled Fixtures And Methods Of Lead Bonding and Deformation. In certain preferred embodiments taught in the ""532 application, the sheet may be stretched by initially attaching it to a ring formed from a material of relatively high coefficient of thermal expansion such as aluminum at a low temperature such as room temperature, then heating the sheet and high-expansion ring and then attaching the sheet to a lower expansion ring such a molybdenum ring. As disclosed, for example, in said U.S. Pat. No. 5,798,286 and in the corresponding PCT International Publication WO 97/11486, the disclosure of which is also hereby incorporated by reference herein, a frame-stretched sheet can be used in other assembly processes using individual semiconductor chips mounted individually to the sheet or mounted on a platen in a preselected array and bonded to the sheet as a unit.
Framed sheets have also been employed in unrelated arts and for different purposes. For example, thin framed sheets referred to as pellicles used in the optical arts as optical beam splitters as shown, for example, in Edmund Scientific, 1997 Optics and Optical Instruments Catalog, p. 56. U.S. Pat. No. 4,037,111 discloses the use of a mechanically stretched sheet held taut by a borosilicate glass frame as a mask for X-ray lithography. German Offenlegungssachrift DE-3,919,564 A1 discloses fabrication of printed circuits by silk-screening onto a polyimide film held taut by an aluminum frame.
U.S. Pat. Nos. 3,537,169; 5,288,663; 5,169,804; 5,654,204; 3,562,058 and 5,362,681 teach processes in which a wafer is adhered to a plastic film or xe2x80x9cdicing tape,xe2x80x9d then sawn into individual chips, whereupon the resulting chips are released from the film. In certain processes described in these patents, the film is carried by a frame.
U.S. Pat. No. 5,622,900 discloses a method in which a wafer is sawn into individual chips prior to completion of fabrication steps. Before sawing, the back side of the wafer is bonded to a xe2x80x9cdicing tapexe2x80x9d held on an opaque, aluminum ring. A UV-transparent xe2x80x9csubstrate waferxe2x80x9d is bonded to the side of the tape opposite from the wafer to be processed. The substrate wafer holds the individual chips in position after sawing. After processing is complete, the assembly is exposed to ultraviolet light through the transparent substrate wafer.
U.S. Pat. No. 5,605,844 discloses a system in which a dicing sheet or film is bonded to a ring of unspecified composition and a wafer is bond to the dicing sheet and sawn into individual chips. After dicing, while the individual chips are still retained on the dicing sheet, the chips are engaged with a tester or xe2x80x9ccontactorxe2x80x9d so that features on the contactor engage features of the chips and/or engage in the kerfs between chips left by the sawing operation. The engaged contactor and chips are then subjected to a burn-in operation at an elevated temperature. As the temperature of the assembly changes, the chips move with the features of the contact and the sheet stretches to accommodate such movement.
Despite all of these improvements and efforts in the art, still further improvements would be desirable.
The present invention addresses these needs.
One aspect of the present invention provides a framed sheet for use in manufacturing a microelectronic component. The framed sheet in accordance with this aspect of the invention includes a flexible sheet having oppositely facing exterior surfaces and a frame having a structure defining an aperture. The structure of the frame includes a light-transmissive portion. The flexible sheet is bonded to the light-transmissive portion of the frame so that the sheet extends across the aperture and so that the frame holds the sheet taut, in tension. The bond between the sheet and the light-transmissive portion of the frame is susceptible to degradation by exposure to light in a degradation wavelength band, and the light-transmissive portion of the frame is adapted to transmit light in such a degradation wavelength band. Thus, the bond between the sheet and the light-transmissive portion of the frame can be released by applying light in the degradation wavelength band to the bond through the light-transmissive portion of the frame. Most preferably, the flexible sheet includes a dielectric layer and at least one, and typically two, metallic layers overlying the dielectric layer at the exterior surfaces. As is further discussed below, the metallic layers can be used to form parts of conductive elements such as leads in the finished component and also serve to stabilize the mechanical properties of the sheet. However, the metallic layers make it impossible to apply degradation light to the bond between the sheet and frame through the flexible sheet. The transparent frame allows application of the degradation light to the bond and thus quickly facilitates removal of the sheet from the frame and reclamation of the frame for reuse. Most preferably, at least the light-transmissive portion of the frame, and preferably the entire frame structure, is formed from a transparent inorganic material. The light-transmissive portion of the frame desirably is formed from a material selected from the group consisting of quartz, fused silica and glasses such as soda lime glass, borosilicate glasses and aluminosilicate glasses. These materials transmit light and also have coefficients of thermal expansion substantially lower than the coefficients of thermal expansion of the materials used for the flexible sheet. This facilitates tensioning of the sheet; provided that the sheet is bonded to the frame at a temperature of above the temperatures encountered in processing, differential thermal contraction of the sheet and frame will tend to maintain tension in the sheet. For certain applications, the frame structure may have a coefficient of thermal expansion approximately equal to the coefficient of thermal expansion of silicon, which facilitates registration of features on the sheet with features on a silicon wafer. Moreover, the most preferred materials have very high elastic module, so that the frame has substantial stiffness even if the structure of the frame is relatively thin. As further discussed below, this facilitates processing operations, such as those which require exposure of the framed sheet to treatment liquids and subsequent removal of the treatment liquids from the framed sheet. The preferred transparent materials, and the use of these transparent materials in conjunction with a light-degradable bond provides a unique combination of benefits.
A related aspect of the present invention provides methods of processing a framed sheet to form microelectronic components. Methods according to this aspect of the invention include the steps of providing a framed sheet including a frame having an aperture and a light-transmissive portion and a flexible sheet extending across the aperture and bonded to the light-transmissive portion of the frame; performing one or more operations on the flexible sheet and then releasing the bond between the sheet and the frame by directing light in a degradation wavelength band through the transmissive portion of the frame through the bond. The operations performed on the sheet may include treatment or formation of features such as conductive leads, vias or other conductive elements to be incorporated in the finished component; formation of features in the dielectric layer which will be included in the finished component such as formation of bonding holes and/or formation of holes for vias and like; and/or engaging features on the sheet with other parts, such as with semiconductor chips, a semiconductor wafer or other parts of the finished microelectronic component. As further discussed below, the tension in the sheet greatly enhances the dimensional stability of the sheet and greatly reduces unpredictable variations in locations of features on the sheet. This facilitates registration of features on the sheet with external elements such as processing equipment or other parts.
In a further aspect of the present invention, the sheet is bonded to the frame using an adhesive activated by light transmitted through the frame. Thus, a method according to this aspect of the present invention includes the steps of providing a frame having an aperture and having a light-transmissive portion adjacent the aperture; temporarily holding a flexible sheet so that the sheet extends across the aperture and overlies the light-transmissive portion of the frame; and providing an adhesive at the interface between the sheet and the light transmissive portion of the frame so that the adhesive contacts the frame and the sheet. During the temporary holding step, the sheet is bonded to the frame by directing light in a curing wavelength band through the light-transmissive portion of the frame so as to cure the adhesive. Most preferably, the temporary holding step includes the step of holding the flexible sheet taut. Typically, the curing wavelength band is in the ultraviolet band. The curing wavelength band may be the same as, or different from, the degradation wavelength band discussed above. In a process which combines aspects of the invention, the sheet may be bonded to the frame using curing light and debonded from the frame using degradation light.
Most preferably, the bonding step is performed while the sheet and the frame are at a temperature or temperatures different from the temperature to be used in processing, and most preferably at an elevated temperature, above the temperatures to be used in processing of the framed sheet, so that differential thermal expansion or contraction occurring between the bonding temperature and the processing temperature make the tension in the sheet at the processing temperature higher than the tension at the bonding temperature.
A further aspect of the invention provides framed sheets for use in manufacturing microelectronic components. The framed sheet includes a frame with a structure defining an aperture. The framed sheet further includes a flexible sheet having oppositely facing exterior surfaces and a first metallic layer at a first such exterior surface. Here again, the sheet extends across the aperture in the frame. The first exterior surface of the flexible sheet is bonded to the structure so that the frame holds the sheet taut. The frame further has at least one contact opening extending through the structure. The sheet includes a contact region aligned with the contact opening, so that the first metallic layer is exposed at the contact opening for engagement with an electrical contact during processing. The bond between the sheet and the structure includes an inner bond region extending outwardly from the aperture of the frame in the vicinity of the contact openings. The inner bond region mechanically isolates the contact region of the sheet from the portion of the sheet which extends across the aperture. Thus, an electrical contact can be engaged with the metallic layer for purposes such as electroplating or electrochemical etching during processing of the framed sheet without disturbing that portion of the framed sheet which is disposed within the aperture. The structure of the frame desirably is formed at least partially from a dielectric material, and most preferably is formed from an inorganic material as discussed above. The aperture may have a polygonal shape such as a rectangle and each contact opening desirably is disposed adjacent a corner of the polygon.
Yet another aspect of the invention provides a framed sheet, again including a frame having a structure defining an aperture and a flexible sheet with oppositely facing exterior surfaces secured to the structure so that the sheet extends across the aperture and the structure holds the sheet taut. According to this aspect of the invention, the flexible sheet includes a dielectric layer and at least one metal layer on at least one surface of the sheet. The at least one metal layer most preferably includes one or more metallic elements having a substantially uniform, distribution of metal within edge regions of the sheet adjacent the edges of the aperture and hence adjacent the structure of the frame. These edge metal regions may include, for example, a continuous layer of metal or a uniform pattern of discrete metallic features. These uniform metallic regions provide uniform structural properties in the edge regions and further promote dimensional stability and predictability in dimensional changes occurring during processing.
According to a further aspect of the invention, a framed sheet may include a frame and a taut sheet in which the metal layer includes a grid of metallic strips defining a plurality of open areas therebetween and extending through a middle portion of the sheet remote from the edges and the structure. In this instance, the metallic strips provide enhanced dimensional stability in the middle regions of the sheet.
A further aspect of the invention provides framed sheets which are particularly well suited to processes involving treatment fluids such as liquids used in electroplating, etching, cleaning and the like. Here again, the framed sheet includes a flexible sheet and a frame having a structure defining an aperture. The structure has a mounting surface with inner edges bounding the aperture and outer edges remote from the aperture. The flexible sheet is bonded to the mounting surface so that the frame holds the sheet taut and so that the sheet extends across the aperture. The sheet desirably is sealed to the mounting surface over substantially the entirety of the edges of the mounting surface, so that processing fluids cannot become lodged between the sheet and the mounting surface. This reduces carryover of processing fluids from one process step to the next, thereby reducing contamination in later stages. Desirably, the sheet is sealed to the mounting surface over essentially the entire mounting surface so that there are essentially no gaps between the surface of the sheet and the mounting surface of the structure.
According to a further aspect of the invention, a framed sheet includes a flexible sheet and a frame including a structure defining an aperture, the structure having a mounting surface facing in a forward direction with an inner edge bounding the aperture. The structure also has an inner edge surface intersecting the mounting surface at the inner edge of the structure and extending in rearward direction away from the mounting surface. Here again, the flexible sheet is bonded to the mounting surface of the structure so that the frame holds the sheet taut and so that a first exterior surface of the sheet overlies the mounting surface of the structure, the sheet being sealed to the mounting surface at least along the inner edge of the mounting surface. Framed sheets, according to this aspect of the invention, most preferably include a fillet joining the first exterior surface of the sheet and the inner edge surface of the frame structure. For example, the fillet may be formed by an adhesive which also bonds the sheet to the structure. The fillet facilitates drainage of processing fluids away from the corner formed by the sheet and the inner edge surface of the structure, thus further reducing carryover of processing fluids. Most preferably, the frame has a rear surface extending generally parallel to the mounting surface, and the rear surface is about 10 mm or less from the mounting surface. For example, the frame may have a uniform thickness of about 10 mm or less. The use of such a thin frame further reduces processing fluid entrapment and carryover. Framed sheets according to the foregoing aspects of the invention may be processed in a method which includes the step of immersing or otherwise exposing the framed sheet to a first treating liquid, removing the article from the first treating liquid and draining the first treating liquid from the framed sheet.
A framed sheet according to a further aspect of the invention also includes a frame assembly with a structure defining an aperture and a flexible sheet secured to the structure and extending across the aperture. The structure of the frame assembly has a neutral plane in bending. The structure holds the sheet in tension, with the sheet being disposed adjacent the neutral plane of the frame assembly. For example, the frame assembly may include a pair of frames disposed on opposite surfaces of the sheet so that the neutral plane of the two frames taken together is coincident with the plane of the sheet. Alternatively, the frame assembly may include a single frame having portions extending on both sides of the sheet. Such a frame may include, for example, a main portion having a mounting surface and a rim portion projecting from such mounting surface so that when the sheet is mounted to the mounting surface, the rim portion is disposed on one side of the sheet whereas the main portion is disposed on the other side of the sheet. Use of a frame assembly with its neutral plane adjacent to the plane of the sheet minimizes any tendency of the frame to bow out of plane due to the forces applied to the frame by the sheet. This further enhances dimensional stability of the sheet.
Yet another aspect of the invention provides methods of bonding a sheet to a frame. In a method according to this aspect of the invention, a flexible sheet is held in engagement with a fixture so that the sheet overlies a top surface of the fixture, with a portion of the sheet overlying a loop-like groove in such top surface. Preferably, the sheet is held to the fixture along a line of engagement outside of the groove. A first surface of the sheet faces away from the top surface of the fixture whereas the second surface of the sheet faces toward the top surface of the fixture. The method further includes the step of applying a differential fluid pressure between the first and second surfaces at the groove as, for example, by applying a subatmospheric pressure within the groove while the first surface remains exposed to atmospheric pressure, so that the fluid pressure on the first surface is higher than the fluid pressure in the groove on the second surface. Thus, differential fluid pressure tends to force the sheet into the groove and pulls a central portion of the sheet to a taut condition. While the central portion of the sheet is taut, a frame is engaged with the central portion of the sheet so that the sheet extends across an aperture in the frame and the frame is secured to the central portion of the sheet, whereupon the sheet is released from the fixture. Most preferably, the step of securing the structure of the frame to the sheet is performed by providing a controlled amount of adhesive at the interface between the frame and the sheet as, for example, by providing a controlled bead of adhesive on the frame or the sheet prior to assembly of these components with one another. The securing step desirably further includes the step of squeezing the frame and the sheet together with a controlled force during the bonding operation. This provides repeatable, controllable beads of adhesive at the edges of the frame so as to form fillets as discussed above.
Still further aspects of the invention provide methods of making framed sheets in which a metallic layer is applied on a surface of a flexible sheet material and in which frames having apertures are applied to the sheet material so that central regions of the sheet material arc exposed through apertures in the frames so that the frames hold the sheet material taut. In methods according to this aspect of the invention, the frame-applying step is performed so that each central region of the sheet is not contacted by any solid object between the time the metal layer is applied on that central region and the time the frame is engaged to surround that central region. Thus, each central region is preserved in its smooth, unwrinkled state up to the time the frame is applied. After the frame is applied, the resulting framed sheet can be handled by engaging the frame, rather than the central region of the sheet. Still other aspects of the invention provide methods of making framed sheets including the step of forming the frame in place on the sheet.
Methods according to still further aspects of the invention use framed sheets having frames with coefficients of thermal expansion different from the coefficient of thermal expansion of the flexible sheet. Plural operations are performed on the framed sheet. The method desirably includes the step of adjusting the temperature of the framed sheet between operations so as to vary the tension of the flexible sheet. Thus, different operations are performed with the flexible sheet under different degrees of tension. For example, a hole-forming operation in which holes are formed in the sheet may be performed while the sheet is under a relatively low tension and other operations may be performed while the sheet is under a tension higher than such relatively low tension. This reduces the possibility that hole-forming operations will cause localized plastic deformation of the sheet.
These and other objects, features and advantages of the present invention will be more readily apparent from the detailed description of preferred embodiments set forth below, taken in conjunction with the accompanying drawings.